1. Field of the Invention
The present invention relates to a charged particle beam irradiation system. More particularly the invention relates to a charged particle beam irradiation system that is preferably applied to a particle beam therapy system for treating an affected part by irradiating the affected part with a charged particle beam such as a proton and a carbon ion.
2. Description of the Related Art
A treatment method which irradiates an affected part of a patient (for example, cancer) with a charged particle beam (ion beam) such as a proton and a carbon ion is known. A charged particle beam irradiation system used for the treatment (a particle beam extraction device or a charged particle beam extraction device) includes an ion beam generator. An ion beam accelerated by the ion beam generator passes through a first beam transport line, and a second beam transport line that is disposed in a rotating gantry, and then reaches an irradiation nozzle disposed in the rotating gantry. The ion beam is extracted from the irradiation nozzle, and the affected part of the patient is then irradiated with the ion beam. As the ion beam generator, for example, a synchrotron (circular accelerator) as described in U.S. Pat. No. 5,363,008 is known. This synchrotron includes: means for causing a charged particle beam to orbit around a closed orbit; means for bringing betatron oscillations of the charged particle beam into a resonance state outside a stability limit of resonance; and an extraction deflector for taking out the charged particle beam from the closed orbit.
An irradiation nozzle shapes the ion beam introduced by the ion beam generator on the basis of the depth from a body surface of the patient and a shape of the affected part, and then irradiates the affected part of the patient on a treatment bed with the ion beam. In general, an irradiation nozzle irradiates an affected part with an ion beam by using any of the following beam irradiation methods: double passive irradiation (non-patent literature 1, p 2081, FIG. 35); Wobbler method (non-patent literature 1, p 2084, FIG. 41); and beam scanning method (Japanese Patent No. 2596292 and non-patent literature 1, p 2092-2093).
An affected part usually has substantial thickness in a traveling direction of an ion beam in a body of a patient. In order to irradiate the affected part with an ion beam over the whole thickness of the affected part, the energy of the ion beam must be controlled so as to form a uniform absorbed dose range, which is wide to some extent, in the traveling direction of the ion beam (that is to say, Spread-Out Bragg Peak; hereinafter referred to as “SOBP”). As energy control means for forming desired SOBP, an irradiation method which adopts Range Modulation Wheel (hereinafter referred to as “RMW”) is proposed (non-patent literature 1, p 2077, FIG. 30). The RMW is a rotational structure in which a plurality of wedge-shaped energy absorbing materials are disposed in a circumference direction so that the thickness of a region through which an ion beam passes changes in point of time. The RMW is so devised that the rotation of the RMW causes the thickness in an ion beam traveling direction (in an axial direction of the RMW) to increase or decrease. Such an irradiation method which uses the RMW is designated as “RMW irradiation method”.
On the other hand, when a charged particle irradiation system is used, there is a possibility that some of charged particle beams extracted from an ion beam generator will not be transported to the charged particle irradiation system. This kind of beams is treated as unnecessary charged particle beams. For this reason, in order to handle the unnecessary charged particle beams, a first beam transport line is provided with a beam dump device (U.S. Pat. No. 5,260,581).
Non-patent literature 1: REVIEW OF SCIENTIFIC INSTRUMENTS, Volume 64, Number 8 (August, 1993) p 2074-2093
Non-patent literature 2: Proceedings of the Symposium on Accelerator and Related Technology for Application, Volume 7 (June, 2005) p 35-36.